1. Field of the Invention
The present invention relates generally to a dual source, power brake booster assembly and more particularly to an assembly having, in series, an air-vacuum operated booster section and a hydraulic pressure operated booster section.
2. Description of the Prior Art
Conventionally, the booster assembly of the above mentioned type has been proposed wherein the booster brake pressures are initially obtained by operation of the vacuum booster section without operating the hydraulic booster section, and are then obtained by operation of the hydraulic booster section after the vacuum booster section reaches its limit or run-out condition. The above assembly is effective to reduce the consumption of vacuum, and an example of such an assembly is illustrated in U.S. Pat. No. 3,935,706.
The hydraulic booster section of such prior art device has, however, differential hydraulic pressure receiving areas which generate a force for biasing a hydraulically actuated power piston, and a valve member operatively associated with an input member so as to actuate the hydraulically actuated power piston, when a hydraulic pressure is generated within the hydraulic booster section. By this construction, another fluidically actuated means, such as, for example, a power steering gear, must be interposed between a hydraulic pressurized source and the hydraulic booster section so as not to generate any hydraulic force within the hydraulic booster section even when the power steering gear is brought into its operative condition.
However, this requires a power steering gear which can deliver at least the fluid pressure required by the hydraulic booster section so as to assure the operation of the hydraulic booster even when the power steering gear is in its operative condition.